Crossing that temperature threshold has all sorts of consequences, for the natural world and people. Low-lying Pacific nations, like Kiribati and the Marshall Islands, likely will be uninhabitable at that level of warming, which is measured as the average temperature increase since the industrial revolution. Storms and heatwaves will get more dangerous. Droughts will become supercharged. Animals will be at risk for extinction, and so on.

That one little number is at the heart of the COP21 U.N. climate talks, which are heading into the home stretch this week here in a Parisian suburb. Over the weekend, negotiators from 195 countries agreed on a draft text of an agreement, whose name is still TBD. The goal is to finalize the draft by Friday, but many, many hurdles remain before that can happen.

Among them: Should 2 degrees remain the world's climate target?

A growing coalition of activists and government officials now are calling for an even lower temperature ceiling: 1.5 degrees Celsius since the industrial revolution.

Read More

Their catchy slogan? "One-point-five to stay alive."

It's not an overstatement. Small island nations long have called for a 1.5-degree threshold, arguing, based on sound science, that they may not exist if temperatures warm 2 degrees because seas will rise too quickly for their islands to adapt.

Now, however, officials in high-polluting countries like France, Canada, Germany and even the United States have started to throw some hip behind the 1.5 movement.

Canada went further. "I support the goals of 1.5," the country's climate minister, Catherine McKenna, said on Sunday, according to a report in the National Observer.

These rhetorical shifts are significant.

But is it scientifically feasible to stop warming short of 1.5 degrees? And, if so, should this more ambitious number become part of international climate policy?

I spent most of Tuesday talking with scientists, researchers and policy makers trying to make sense of those two questions. The answers, from where I sit: Yes and hopefully.

Our global ambition should be to stop warming short of 1.5 degrees.

That's what the latest science calls for, and it's also the moral thing to do. Otherwise, our indifference and our addiction to dirty energy could wipe small countries off the map.

I'd like to see the 1.5 target remain in the draft of the COP21 agreement in Paris, but I also don't think negotiators and activists should become so fixated on 1.5 degrees that they derail a deal. The actions we need to take to stop warming short of 1.5 degrees or 2 degrees are remarkably similar. In both cases, another number is crucial: zero.

The world needs to move rapidly toward zero net carbon emissions. To hit the 1.5 degree goal, that needs to happen sometime between about 2030 and 2050, according to the researchers I spoke with. For 2 degrees, there's a little more time, perhaps until 2050 to 2080. Either way, the strategy is the same: Move off fossil fuels absolutely as quickly as possible. Doing so -- and doing so now -- ultimately will cost less in terms of overall social and environmental effects.

The fight between 1.5 and 2 degrees shouldn't obfuscate that clear reality.

"The main point is you have to go to zero quite rapidly," said Glen Peters, a senior researcher at the Center for International Climate and Environmental Research in Oslo, Norway.

And since world leaders already have agreed -- long before Paris -- to try to limit warming to 2 degrees Celsius, continuing to chase that goal shouldn't be seen as a disappointment to environmentalists or small-island states. These targets, according to many researchers, including the Yale economist who first proposed the 2 degrees target in the 1970s, help us estimate the "danger zone" for climate change. But it's not like the world falls off a cliff at 1.51 degrees of warming. Our predictions aren't that good. All climate change policy is about gambling. Disaster becomes more likely the warmer the atmosphere gets.

What's clear: We must do as much as we can.

The most important outcome of a successful agreement here in Paris, as many have argued, could be sending the signal to leaders in business, investment, government and media that the world is moving away from dirty energy sources, quickly, and isn't turning back.

Both 1.5 and 2 degree targets send that message.

Stopping warming short of 1.5 or 2 degrees will be extremely difficult, but it's possible. "From the point of view of science, technology and economics, the literature and modeling on energy and climate systems shows that it's feasible to limit warming to below 1.5 degree by 2100," said Bill Hare, the physicist who is founder and CEO of Climate Analytics.

Shooting for the 1.5-degree ceiling requires more faith in technology, he told me, since we'll need to find a way to vacuum carbon out of the atmosphere in order to meet it. We're almost certain to blow past 1.5 degrees of warming (we're already at 1 degree) this century based on the pollution we've already put into the atmosphere, researchers told me, but they're hoping we might be able to bring temperatures back down, boomerang style, with new tech.

There's a certain danger in hoping technology will save the day, but recent announcements by Bill Gates and others, who plan to put billions into energy research and development, are cause for optimism. Meanwhile, according to Mark Jacobson, a professor of civil and environmental engineering at Stanford University, we know how to get to 100% renewable energy by 2050 using even just the technologies we have right in front of us.

"The answer is yes, it's possible," he said. "Will anyone do it is another question."

A global tax on carbon would help push us in the right direction.

As would bold policies at all levels of government to bolster the transition.

Stopping warming short of 1.5 degrees is a laudable goal. The fate of small island states, like the Marshall Islands, which I visited earlier this year, may hang in the balance between those two numbers. As could the long-term fate of the Greenland ice sheet, which would cause disastrous levels of sea rise worldwide if it melts over the course of centuries.

But the number isn't as important as the trajectory.

I want to be able to look into the eyes of people in the Marshall Islands -- a beautiful, fragile place, where the word for hello ("iakwe") also means "I love you" and "You are a rainbow" -- and say that I did everything I could to help save their land, language and culture.